A silicide layer is formed on a semiconductor region such as source/drain regions and a polysilicon gate electrode of a MOS transistor to form low resistance contact. Silicide forming metal includes titanium, tungsten, molybdenum, cobalt, nickel and the like. Alloy silicide has also been studied. Nickel silicide among others has drawn remarkable attention because nickel silicide has the advantage that even a fine gate electrode has stable resistance.
Nickel silicide added with metal such as Pt for improving heat resistance is formed by a silicidation process using an Ni alloy film made of Ni and metal such as Pt.
In a silicidation process using an Ni alloy film, an Ni alloy film is first deposited on a gate electrode and source/drain diffusion layers. Next, if necessary, a protective film of titanium oxide (TiN) or the like is deposited on the Ni alloy film.
Next, first silicidation heat treatment is performed at a relatively low temperature of 300° C. or lower to form a silicide ((Ni alloy)2Si) film having a composition ratio of 2:1 between Ni alloy and Si on the gate electrode and source/drain diffusion layers.
Next, the protective film and an unreacted portion of the Ni alloy film is selectively removed by wet etching. Second silicidation heat treatment is performed at a relatively high temperature of 300° C. to 400° C. or lower to form a nickel alloy mono silicide film having a low resistance on the gate electrode and source/drain diffusion layers.
Applied Surface Science 73 (1993)197 reports a silicidation process of dilute Ni95Pt5 alloy mixing Pt 5% to Ni. Ni segregates first from alloy to underlying Si to form Ni2Si at an alloy-Si interface. At this stage almost all Pt is left in the alloy. With this Ni supply, Ni in a region of the alloy near silicide is removed, and this region changes gradually to a Pt-rich zone. As all Ni atoms react, a portion of Pt diffuses into Ni2Si, reaches and is accumulated at the Si surface to form silicide. Thereafter, Ni2Si is transformed to NiSi. There is a tendency that Pt moves from the silicide-Si surface to an outer surface. As anneal continues further at a high temperature, Pt distributes again.
J. Appl. Phys., 84 (1998)2583 reports that heat resistance is improved by adding platinum (Pt) to nickel silicide.
JP-A-2008-78559 reports that an Ni—Pt alloy layer is formed on an Si substrate by sputtering, primary heat treatment is performed at a temperature of 240° C. to form Ni2Si through silicidation reaction of Ni—Pt alloy and Si, an unreacted Ni—Pt alloy layer is removed, and secondary heat treatment is performed at a temperature of 400° C. to form nickel platinum mono silicide. It is reported that chemical resistance is improved more than without Pt and surface roughness is suppressed. It analyzes that silicide of Pt-rich Ni—Pt alloy is formed on the surface.
JP-A-2008-118088 proposes that after primary heat treatment is performed at a relatively low temperature of 300° C. or lower to react an Ni—Pt film and an underlying silicon surface, an unreacted portion is selectively removed with chemical containing hydrogen peroxide at a temperature of 71° C. or higher, an oxide film being formed on the surface of an Ni—Pt—Si film, and thereafter secondary heat treatment is performed at a relatively high temperature of 300° C. to 400° C. to form a nickel alloy mono silicide film.
JP-A-2007-324187 reports that after an Al film is deposited on an Si surface, heat treatment is performed to form an Al-containing Ni silicide layer, and thereafter heat treatment is performed at, e.g., 650° C. It is reported that Al distributes mainly in an uppermost surface region of the Ni silicide layer. Although an Ni silicide layer on an n-type Si layer has heat resistance lower by about 100° C. than that of an Ni silicide layer on a p-type Si layer, the Al-containing Ni silicide layer demonstrates similar heat resistance both to n-type and p-type Si layers.
The characteristics of additive-containing Ni silicide (NiSi) of an Si device are not still analyzed sufficiently. It is said that NiSi grows in a spike shape. If a top end of a spike reaches near the junction surface, a junction leak current is considered to be increased.
An NiSi layer on an Si surface forms a Schottky contact. As Al is added to NiSi, it is said that a barrier height of Schottky contact lowers. As Pt is added to NiSi, it is said that heat resistance and chemical resistance are improved.